Re: [PATCH v2 3/4] mm: page table check

On Tue, Dec 7, 2021 at 7:05 PM Andrew Morton [off-list ref] wrote:

On Sat,  4 Dec 2021 18:23:13 +0000 Pasha Tatashin [off-list ref] wrote:

quoted

Check user page table entries at the time they are added and removed.

Allows to synchronously catch memory corruption issues related to double
mapping.

When a pte for an anonymous page is added into page table, we verify that
this pte does not already point to a file backed page, and vice versa if
this is a file backed page that is being added we verify that this page
does not have an anonymous mapping

We also enforce that read-only sharing for anonymous pages is allowed
(i.e.  cow after fork).  All other sharing must be for file pages.

Page table check allows to protect and debug cases where "struct page"
metadata became corrupted for some reason.  For example, when refcnt or
mapcount become invalid.

...

--- a/arch/Kconfig
+++ b/arch/Kconfig

@@ -1307,6 +1307,9 @@ config HAVE_ARCH_PFN_VALID
 config ARCH_SUPPORTS_DEBUG_PAGEALLOC
      bool

+config ARCH_SUPPORTS_PAGE_TABLE_CHECK
+     bool
+

I guess a dependency on CONFIG_DEBUG_VM would be appropriate?

I do not think CONFIG_DEBUG_VM is needed here. We would like to have
page table check enabled on some production machines to ensure there
are no double mappings. With CONFIG_DEBUG_VM enabled that would not be
possible. For example, CONFIG_PAGE_OWNER, also uses extended struct
page entries and does not depend on CONFIG_DEBUG_VM

Pasha